https://github.com/mootools/mootools-core/blob/master/Source/Element/Element.js#L629-632
I believe that IE is considering any non-normative attribute (data-*) as an
expando. Those lines were added to prevent custom methods from being
returned. Looks like my spec coverage was too simple.
For now you can workaround this with:
``` javascript
Element.Properties['data-role'] = {get: function(){
return this.getAttribute('data-role');
}};
```
(I know) Not a great solution, since you'll need to repeat the above for
each custom property that you've set. But it's temporary for 1.4.4.

This fixes a nasty regression that custom attributes set by HTML text
(e.g. innerHTML) would previously be considered `expando` and therefore
thought to have been considered a fake attribute. Therefore, we didn't
return the value.
This fix relies now on outerHTML to check for the existence of the
attribute. Keep in mind this also fixes the previous bug of returning
custom functions since any new `el.attribute =` are not shown in
outerHTML.
PASSED: IE6-9.

This fixes a nasty regression that custom attributes set by HTML text
(e.g. innerHTML) would previously be considered `expando` and therefore
thought to have been considered a fake attribute. Therefore, we didn't
return the value.
This fix relies now on outerHTML to check for the existence of the
attribute. Keep in mind this also fixes the previous bug of returning
custom functions since any new `el.attribute =` are not shown in
outerHTML.
PASSED: IE6-9.

this is still not quite right. http://jsfiddle.net/dimitar/357Lt/ -> a data-propertyId is arguably wrong as HTML5 validation prefers all data- properties to be lowercase (W3C spec does not say anything in particular about their casing).

Browsers seem to adjust the element and fix attributes to lowercase but IE6/7/8 do not.

anyway: in IE7/8, .get('data-propertyid') - which is correct from a getter stand point as it SHOULD be lowercase - returns null though .getAttribute('data-propertyid') returns the value, despite of the element markup having data-propertyId.